Turbochargers are provided on an engine to deliver air to the engine intake at a greater density than would be possible in a normal aspirated configuration. This allows more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight.
Generally, turbochargers use the exhaust flow from the engine exhaust manifold, which enters the turbine housing at a turbine inlet, to thereby drive a turbine wheel, which is located in the turbine housing. The turbine wheel is affixed to one end of a shaft, wherein the shaft drives a compressor wheel mounted on the other end of the shaft. As such, the turbine wheel provides rotational power to drive the compressor wheel and thereby drive the compressor of the turbocharger. This compressed air is then provided to the engine intake as referenced above.
In designing the turbine stage, selection of the turbine stage components is made relative to a preferred performance point. In a simple uncontrolled fixed-nozzle turbocharger system, an uncontrolled turbocharger is designed so that optimal performance is reached at high engine speeds. However, at other speeds the turbocharger provides suboptimal boost or air volume to the engine.
Controlled turbochargers provide improved performance, in that the turbine optimal operating point is already reached at low or medium engine speeds. Generally in a controlled system, when the flow rate of exhaust gases increases and the turbocharging pressure becomes too high, part of the exhaust gases are discharged into the surrounding atmosphere through a wastegate so as to bypass the turbine.
Typically, the exhaust gas flows through a volute defined within the turbine housing or casing. Further, a wastegate passage is also provided which is separated from the volute by an intermediate wall. To provide for wastegate flow, a wastegate port is provided in the wall which port is controlled by a wastegate valve.
The wastegate valve is selectively openable and closable during operation of the turbocharger. In one known wastegate valve, a flap type valve is provided which has a valve body having a valve face which faces toward and abuts against a valve seat when the valve body is in a closed position. The valve face and an opposing face of the valve seat typically conform to each other and have rigid, complementary shapes, which are intended to provide a tight seal. However, an optimum seal may be prevented if the ball and seat faces do not conform well to each other, or are misaligned. For example, there may be geometric misalignment due to misalignment tolerances present during assembly, and also due to thermodynamic expansion or contraction during operation of the turbocharger.